The present invention relates to an electric power steering apparatus.
The electric power steering apparatus assists steering force through motor driving in order to realize an excellent steering feeling.
Specifically, the electric power steering apparatus disclosed in Japanese Laid-Open Patent Publication No. 2006-131191 has an assist torque calculating unit which calculates assist torque based on a vehicle speed and steering torque. The assist torque calculating unit increases the assist torque as the steering torque becomes larger. The assist torque calculating unit also decreases the assist torque as the vehicle speed V becomes faster. A steering assist force according to this calculated assist torque is applied to the steering shaft through the motor.
The range of movement of a steerable wheel is defined in a general vehicle. Specifically, a rack shaft is moved in an axial direction along with operation of the steering wheel, whereby the wheel is turned. The movement of this rack shaft is restricted by its end portion brought into an end touching state of touching against a rack housing. Thus, steering beyond the range of movement of the steering wheel is restricted.
Electric power steering apparatus disclosed in Japanese Laid-Open Patent Publication No. 2003-312514 and Japanese Laid-Open Patent Publication No. 2008-260421 determine the end touching state in the electric power steering apparatus if the steering torque exceeds a threshold due to the steering wheel being further steered to the end side from the end touching state. In this case, a process of suppressing heat generation in a circuit or an impact absorbing process such as suppressing the assist torque is carried out. The impact absorbing process suppresses a load applied to a steering shaft, particularly to an intermediate shaft accompanying the assist torque applied in the end touching state.
However, the electric power steering apparatus described in the above documents determine that the end touching state has been reached when the steering torque exceeds a threshold, that is, after an axial force more than a fixed value is generated in the rack shaft. Therefore, the suppression of the assist torque is carried out after a fixed load is applied to the intermediate shaft. In terms of suppressing the load applied to the intermediate shaft, in terms of suppressing the heat generation in the circuit, it has been demanded to more quickly make the determination of the end touching state.